Method for surface treatment of a die-casting die

ABSTRACT

The present invention is to provide a method for surface treatment that substantially provides no nitride compound layer that causes heat checks and abrasion to a die, while nitride is introduced in large quantities into the die internally, and as a result a die-casting die with excellent heat check resistance and excellent abrasion resistance can be produced. The method comprises a step of a nitriding process for forming on an aesthetic surface of the die-casting die a nitrided layer that includes at least a compound layer composed of a nitrogen compound by introducing gas containing at least ammonia gas to a heating furnace, a step of decomposing the compound by exhausting the ammonia gas from the heating furnace and for introducing ambient gas to the heating furnace, to carry out a thermal process to decompose the nitrogen compound, and a step of processing a shot peening process on the aesthetic surface of the die. The thickness of the compound layer contained in the nitrogen layer that is formed in the step of nitriding process is within the range of from 2∫ m to 7∫ m.

TECHNICAL FIELD

This invention relates to a method for the surface treatment of adie-casting die is provided by applying the compression residual stressin an aesthetic surface of the die-casting die using a shot-peeningprocess.

BACKGROUND OF THE INVENTION

In a die-casting molding in which a casting cycle that includes pouringa molten metal, solidification, and stripping a cast product isrepeated, small heat checks on an aesthetic surface of a die-casting thecan be readily produced by a thermal history to be given by the castingcycles. Thus abrasions caused by mechanical contacts can be readilyproduced. The heat checks may develop into a crack or cracks to damagethe die, and the abrasion may degrade the dimensional accuracy of thecast product. Therefore, to enhance heat check resistance and abrasionresistance to extend a product's life, processes such as a surfacenitriding process-to enhance the hardness of the aesthetic surface ofthe die and a shot-peening process to provide the compression residualstress on the aesthetic surface are carried out.

The surface nitriding process in the die is often carried out by a gasnitriding process. The primary reasons for this are in the aspects ofease and the cost of the process. The gas nitriding process decomposesammonia gas under a high temperature and diffuses the generated nitrogenfrom the aesthetic surface of the die to inside the die such that adiffusion-hardening layer is provided.

The shot-peening process at the die is mostly carried out byaccelerating small spheres consisting of a ceramic or a hard metal lessthan 1 mm in diameter using a projection device to project them at theaesthetic surface of the die. On the aesthetic surface of the die, thecompression residual stress is achieved based on the hardening processby means of the collisions of the small spheres.

For example, Patent Literature 1 discloses that the aesthetic surface ofthe die is first processed by the surface nitriding process to form anitrogen diffusion-hardening layer and is then processed by theshot-peening process to yield a high compression residual stress on thesurface of the diffusion-hardening layer. The surface nitriding processand the shot-peening process are carried out in combination such thatthe product lifetime of the die may be significantly extended.

It is known for the nitriding process that a compound layer lacking aplastic deformability is formed on the surface of the nitrogendiffusion-hardening layer. Because such a compound layer causes the heatchecks to develop into a crack or cracks and abrasion due to a spalling,a nitriding process is proposed to prevent the formation of the compoundlayer, or to form it as thinly as possible.

For instance, Patent Literature 2 disclose a two-step process in whichan ammonia gas nitriding process is first carried out in a range ofrelatively low temperatures of 450-530° C. The supply of the ammonia isthen reduced or disrupted, while the nitrogen is internally diffused ata range of processing temperatures of 550-590° C. Generally, with theammonia gas nitriding process under the range of relatively lowtemperatures, the compound layer is thinly formed. However, the depth ofthe nitrogen diffusion layer also becomes shallow. Therefore, thenitrogen of the nitrogen diffusion layer is deeply diffused in the dieto provide the thick nitrogen diffusion layer, while the thin compoundlayer is maintained.

Similarly, Patent Literature 3 disclose a two-step process in which anammonia gas nitriding process is first carried out under reducedpressure at a temperature less than 570° C. The supply of the ammonia isthen reduced or disrupted, while the nitrogen is internally diffused ata range of processing temperatures of 570-650° C. Patent Literature 3describes that the nitride compound layer can be thin and in anon-porous state, while the depth of the nitride compound layer can bedeeper than that of the nitrogen diffusion layer by the thermal process.

PRIOR-ART DOCUMENTS Patent Literature

-   [PATENT LITERATURE 1] Japanese Patent Laid-open Publication No.    2004-148362-   [PATENT LITERATURE 2] Japanese Patent Laid-open Publication No.    Tokkai-Hei 10-306364-   [PATENT LITERATURE 3] Japanese Patent Laid-open Publication No.    Tokkai-Hei 11-100655

SUMMARY OF THE INVENTION The Problem to be Solved by the Invention

As disclosed in Patent Literature 2 and 3, in the ammonia gas nitridingprocess to form the thin nitride compound layer, because actualquantities of the nitrogen supplied to the die are little, the nitridecompound layer cannot have a sufficient hardness when one makes anattempt in which the nitrogen of the nitrogen diffusion layer is furtherdeeply diffused in the die by the thermal process.

The present invention was accomplished in view of such circumstances.The object of it is to provide a method for surface treatment thatsubstantially provides no nitride compound layer that causes the heatchecks or the abrasion to the die, while it can internally introducelarge quantities of the nitride into a die. This results in producing adie-casting die with an excellent heat check resistance and an excellentabrasion resistance.

Means to Solve the Problem

The inventors of* the present invention found that the uppermost layer,composed of the nitride compound that is formed by various nitridingprocesses, such as a gas nitro carburizing process, a gassulpho-nitriding process, and a plasma nitriding process, can be readilydecomposed by a thermal process. By a study about providing a method formanufacturing a die that substantially provides no nitride compoundlayer to the die, the inventors thought that the above decompositiongenerates nitrides such that the quantities of the nitrides supplied tothe die may be increased by diffusing the generated nitrides in the dieinternally.

Accordingly, the method for the surface treatment of a die-casting diethat is provided by applying the compression residual stress in anaesthetic surface of the die-casting die using a shot peening process,of the present invention, comprises the step of nitriding processing forforming on an aesthetic surface of the die-casting die a nitrided layerthat includes at least a compound layer composed of a nitrogen compoundwith a nitriding process, for instance, gas nitro-carburizing, gassulpho-nitriding, and plasma nitriding, by introducing gas containing atleast ammonia gas to a heating furnace, the step of decomposing thecompound for exhausting the ammonia gas from the heating furnace and forintroducing ambient gas to the heating furnace, to carry out a thermalprocess to decompose the nitrogen compound, and the step forshot-peening the aesthetic surface of the die. The thickness of thecompound layer contained in the nitrogen layer that is formed in thestep of the nitriding process is within the range of from 2 μm to 7 μm.

With this method, the thickness of the nitrogen compound layer, which isformed with the nitriding process by introducing the gas containing atleast ammonia gas to the heating furnace, is controlled to thepredetermined thickness such that the nitrogen compound is decomposed inthe step of decomposing the compound. As a result, no nitrogen compoundlayer is substantially provided. Thus the nitrogen is instead generatedto increase quantities of the nitrogen to be supplied to the die suchthat a nitrogen diffusing layer having high hardness can be provided.Despite this, the substantially denied nitrogen compound layer shouldremain as the layer contains a significant number of voids to absorb anddisperse the collision energies of the shots in the shot-peeningprocess. However, the thickness of the nitrogen compound layer is alsocontrolled to be at the predetermined thickness in the nitriding stepsuch that the compression residual stress by the shot-peening processcan be imparted on the nitrogen diffusion layer.

Therefore, based on the high hardness and the high compression residualstress, a die-casting die having excellent abrasion resistance andexcellent heat check resistance can be produced.

In the above method, the step of decomposing the compound preferablycarries out the thermal process at a temperature that is lower than thatof the step of the nitriding process. In this case, higher hardness andhigher compression residual stress can be imparted on the die-castingdie such that the the having more excellent abrasion resistance and moreexcellent heat check resistance can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one specimen to be used with theembodiment of the method for surface treatment of the die-casting die ofthe present invention.

FIG. 2 indicates the steel classes and the conditions of the surfacetreatment of the specimens disclosed in FIG. 1.

FIG. 3 illustrates one example of the heating and cooling examinationequipment to be used with the embodiment of the method for surfacetreatment of the die-casting die of the present invention.

FIGS. 4( a) and 4(b) are enlarged sectional views illustrating thechanges of the specimen near the surface after the nitricling step (FIG.4( a)) and after the compound decomposition step (FIG. 4( b)) in themethod for surface treatment of the die-casting die of the presentinvention.

FIGS. 5( a) and 5(b) are enlarged sectional views illustrating thechanges in the nitrogen density of the specimen near the surface ofFIGS. 4( a) and 4(b).

FIG. 6 is a graph indicating the relationship between the thickness ofthe compound layer and the number of heat checks (HC) on the specimen onwhich the method for surface treatment of the die-casting die of thepresent invention has been applied.

FIG. 7 is a graph indicating the relationship between the thickness ofthe compound layer and the residual stress on the specimen on which thestep of shot peening of the method for surface treatment of thedie-casting die of the present invention has been applied.

FIG. 8 is a photograph of the cross section of the specimen (theeleventh embodiment) after the nitriding step.

THE EMBODIMENTS TO CARRY OUT THE INVENTION

On the method for surface treatment of the die-casting die of thepresent invention, the details will be explained with reference to theresult of the verification test as indicated in FIGS. 1 to 8. In theverification tests, cylindrical specimens 1 (see FIG. 1) eachcorresponding to a die-casting die are prepared and various surfacetreatments are then carried out on them to evaluate them.

Cylindrical specimens 1 having an outer diameter D1=15 mm, an innerdiameter D2=3 mm, and a length L=20 mm are prepared. Each specimen 1 ismachined from a round bar stock of Japanese Industrial Standards (JIS),the steel material of an alloy toll SKD 61 equivalency. Instead of theSKD 61 equivalency, the specimens 1 in the ninth embodiment and thetenth embodiment are machined from round bar stocks of JIS SKD 7equivalency and a steel material of an alloy toll SKH51 equivalency. Thesteel classes of the embodiments and comparison examples are describedand compiled in FIG. 2.

Each specimen 1 is then heated in a heating furnace while ammonia gas isintroduced in the heating furnace such that the peripheral surface ofthe specimen is processed by a gas nitro-carburizing process (thenitriding step). Instead of the gas nitro-carburizing process, thespecimen of the sixth embodiment is processed by a gas sulpho-nitridingprocess. Also, the specimens of both the seventh embodiment and thefifth comparison example are processed by a plasma-nitriding process.The types of nitriding processes, the types of gasses, the temperatures,and the times of the embodiments and the comparison examples aredescribed and compiled in FIG. 2.

The ammonia gas is then exhausted from the heating furnace, followed bywhich nitrogen is introduced as an atmospheric gas to the heatingfurnace, while the specimen is still thermal processed therein such thatit is diffusion-processed. Thus, a nitrogen compound within a compoundlayer 2 (see FIG. 4) that is produced by the nitriding process, asdescribed below, is completely dissolved (the step for decomposing thecompound). The temperatures and the times of the diffusion process arealso described and compiled in FIG. 2.

Spheres, for instance, each composed of an amorphous alloy of 0.05mm-0.2 mm in diameter, are projected and shot-peened on the peripheralsurface of the specimen 1 with projection pressure of 0.3 MPa (the stepfor shot-peening).

On the specimen 1 that has been processed by the above process, theresidual stress of the peripheral surface near the longitudinal centeris measured.

Further, as illustrated in FIG. 3, each specimen 1 is tested by heatingand cooling with examination equipment 20 repeatedly such that the heatcheck resistance is evaluated. More particularly, a small diameterportion 22 a of a supporting member 22 of the examination equipment 20is inserted through hole 1 a in the specimen 1 such that the specimen 1is pinched by means of a holder 23 from the top and bottom to fix it.The peripheral surface of the specimen 1 is heated from room temperatureto 700° C. with a high frequency coil 21 in four seconds. Coolant water24 is then jetted from a jet orifice (not shown) to cool the specimen 1to room temperature in :three seconds. The heated and cooled specimen 1is then dried with an air blow for one second. This cycle of heating,cooling, and drying is repeated 1,000 times in total and the specimen 1is removed from the examination equipment 20. The specimen 1 that isremoved from the examination equipment 20 is cut near the longitudinalcenter in the plane that is perpendicular to the center axis thereof.The cut specimen 1 is then mounted on a resin to mirror polish the cutsurface of it. The cut surface of that specimen is observed using anoptical microscope of 100× magnification, to count the number of heatchecks (HC) that appear on the peripheral surface of the specimen 1.

In addition, any pieces of the specimens 1 are removed from the heatingfurnace after the nitriding process described above to measure thethickness of the compound layer 2 (see FIG. 4) as below described. Thespecimen 1 that is removed from the heating furnace is cut near thelongitudinal center in the plane that is perpendicular to the centeraxis thereof. The cut surface of the cut specimen 1 is then mirrorpolished. The polished cut surface is then observed using an opticalmicroscope to measure the thickness of the compound layer 2.

In the nitriding process, as illustrated as in FIGS. 4( a) and 5(a), theactivated nitrogen in a gas phase diffuses from the peripheral surfaceof the specimen 1 to the inside (a substrate) 4 to form a nitride layer5 near the peripheral surface. The nitride layer 5 consists of anitrogen compound layer 2 that is the uppermost layer and a nitrogendiffusion layer 3 on the inside the former. The compound layer 2consists of a complex nitride of Fe or Cr, and is an extremely fragilelayer. It is noted that the growing rate of the compound layer in theplasma-nitriding process is significantly lower than that of thegas-nitriding process. The nitrogen diffusion layer 3 is a solidsolution layer of the nitrogen including a nitride that is dispersed andprecipitated.

In the diffusion process following the nitriding process describedabove, the depth of the nitride layer 5 is increased, as illustrated inFIGS. 4( b) and 5(b). More particularly, a flux in the nitrogen suppliedto the specimen 1 through the peripheral surface from the gas phase isdegraded such that the nitrogen of the nitrogen diffusion layer 3primarily diffuses to the inside of the specimen 1. In this time, if thenitrogen compound in the compound layer 2 dissolves to yielded nitrogen,the yielded nitrogen also diffuses to the inside of specimen 1. Becausethe density of the nitrogen contained in the compound (see 3 a in FIG.5( a)) is significantly higher than that contained in a solid solution(see 3 b in FIG. 5( b)), e.g., the nitrogen diffusion layer 3, of thenitrogen, the nitrogen diffusion layer 3 that contains significant largequantities of the nitrogen can be obtained (see 31 in FIG. 5( b)). InFIG. 5( b), 32 denotes the case in which only the nitrogen diffusionlayer 3 provided by only the nitriding process is diffusion processed.

On the other hand, if the complex nitride in the compound layer 2dissolves, a shrinkage in its volume results in a surface layer 2′ thatcontains a large void volume. Such a surface layer 2′ absorbs anddissipates the collision energies of the shots in the shot-peeningprocess such that the formation of the compression residual stress isinhibited. Details of it are described below.

The results of the measurement described above will be now explained.FIG. 6 denotes the relationship between the thickness of the compoundlayer 2 and the number of the heat checks (HC) on the specimen after therepeated heating and cooling tests.

It is found that the number of heat checks can be decreased as thethickness of the compound layer 2 is increased such that the heat checkresistance may be enhanced. Indeed, forming a thin compound layer 2 of1.5 μm in the first comparison example provides 597 heat checks and of1.0 μm in the fifth comparison example provides 441 heat checks. Incontrast, forming a thick compound layer 2 of 2 μm to 7 μm in the firstthrough fourteenth embodiments provides significantly reduced numbers,13 through 257, of heat checks.

Particularly, the heat check number significantly decreased in theeleventh embodiment in which the diffusion process (the heatingtreatment) is carried out at a temperature that is lower than that ofthe nitriding process. Therefore, it can be found that the heattreatment in the step for compound decomposition preferably is carriedout at a temperature lower than that of the nitriding step.

Because the quantities of the nitrogen compound to be decomposed by thediffusion process can be increased as the thickness of the compoundlayer 2 is increased, as described above, the quantity of nitrogen inthe nitrogen diffusion layer 3 can be increased, the hardness after thediffusion process can be enhanced to improve the wear resistance, andthe heat check resistance can be improved.

On the other hand, it can also be found that the number of heat checksrapidly increase as the thickness of the compound layer 2 is increasedabove a given thickness such that the heat check resistance may besignificantly degraded. Indeed, forming thick compound layers 2 of 8.0μm, 9.0 μm, and 10.0 μm in the first, second, and fourth comparisonexamples provide 706, 707, and 840 heat checks, respectively. Theyrapidly increased in comparison with those of the first throughfourteenth embodiments.

In association with the above, FIG. 7 graphs the relationship betweenthe thickness of the compound layer 2 and the residual stress to begiven to the specimen 1 by the shot-peening. In FIG. 7, the compressionstresses is expressed in negative values.

It can be found that the absolute value of the compression residualstress may be increased as the thickness of compound layer 2 isincreased. Indeed, forming thin compound layers 2 of 1.5 μm and 1.0 μmin the first and fifth comparison examples provides −965 MPa and −993MPa of the compression residual stress. In contrast, forming a thickcompound layer 2 of 2 μm to 7 μm in the first through fourteenthembodiments provides significant increased compression residual stress,−1350 MPa through −1755 MPa. They are significantly increased incomparison with those of the first and fifth comparison examples.

On the other hand, it can also be found that the absolute value of thecompression residual stress may be rapidly degraded as the thickness ofcompound layer 2 is increased above a given thickness. Indeed, formingthick compound layers 2 of 8.0 μm, 9.0 μm, and 10.0 μm in the second,third, and fourth comparison examples provide compression residualstress of −1298 MPa, −1251 MPa, and −938 MPa, respectively.

As described above, increasing the thickness of the compound layer 2above a given thickness causes that the absolute value of thecompression residual stress is significantly decreased and thus the heatcheck resistance is significantly degraded. This is due to the surfacelayer 2′ (see FIG. 4( b)) which contains large quantities of voids thatare formed by decomposing the compound of the compound layer. Morespecifically, forming a thick compound layer 2 results in a thicksurface layer 2′ by the diffusion process such that the formation of thecompression residual stress by shot-peening is rapidly inhibited, andthus the heat check resistance is significantly degraded.

FIG. 8( a) is an optical microscope photograph of the cutting surface ofthe specimen 1 after the nitriding process in the eleventh embodiment.FIG. 8( b) is an optical microscope photograph of the cutting surface ofthe specimen 1 after the diffusion process following the nitridingprocess in the eleventh embodiment. In the former, the compound layer 2and the nitrogen diffusion layer 3 can be observed. In the latter, theincreased thickness of the nitrogen diffusion layer 3 and the surfacelayer 2′ that contains the voids blackly, especially near the nitrogendiffusion layer 3, by decomposing the compound, can be observed.

Based on the foregoing descriptions, constantly limiting the thicknessof the compound layer 2 and thus the surface layer 2′ that contains thevoids, which remain after the compound is decomposed, provides thecompression residual stress with an excellent property by shot-peeningsuch that a die-casting die with an excellent heat check resistance canbe obtained. Particularly, under the general conditions of theshot-peening process such as in the embodiments, the thickness ofcompound layer 2 (the surface layer 2′ remaining after the compound isdecomposed) to achieve this purpose is 2-7 μm.

The relationships between the thickness of the compound layer 2 and thenumber of heat checks or the compression residual stress in FIGS. 6 and7 line up to the same plotted lines, regardless of differences betweentypes of nitriding processes, for instance, gas nitro-carburizing, gassulpho-nitriding, and plasma nitriding such as in the sixth and seventhembodiments. Also, they are regardless of differences between the steelmaterials of SKD61 equivalency material, SKD7 equivalency material, andSKH51 equivalency material in the ninth and tenth embodiments. Namely,these results suggest that the method of the present invention can beapplied regardless of the differences between types of nitridingprocesses and the differences between the common materials for the die.

As discussed above, the embodiments of the method for surface treatmentof a die-casting die of the present invention comprises the step of anitriding process for forming on the aesthetic surface of thedie-casting the a nitrided layer that includes at least the compoundlayer composed of a nitrogen compound with the nitriding process, forinstance, gas nitro-carburizing, gas sulpho-nitriding, and plasmanitriding, by introducing gas containing at least ammonia gas to theheating furnace, the step of decomposing the compound for exhausting theammonia gas from the heating furnace and for introducing ambient gas tothe heating furnace, to carry out the thermal process to decompose thenitrogen compound, and the step of the shot-peening for the shot-peeningprocess on the aesthetic surface of the die. The thickness of thecompound layer contained in the nitrogen layer that is formed in thestep of the nitriding process is within the range of from 2 μm to 7 μm.

The thickness of the nitrogen compound layer, which is formed with thenitriding process by introducing gas containing at least ammonia gas tothe heating furnace, is kept at the predetermined thickness such thatthe nitrogen compound is decomposed in the step of decomposing thecompound. At a result, no nitrogen compound layer is substantiallyprovided and thus nitrogen is instead generated to increase the quantityof nitrogen to be supplied to the die such that a nitrogen diffusinglayer having high hardness can be provided. Despite this, thesubstantially denied nitrogen compound layer remains as a layercontaining a significant volume of voids to absorb and disperse thecollision energies of the shots in the shot-peening process. However,the thickness of the nitrogen compound layer is also kept at thepredetermined thickness in the nitriding step such that the compressionresidual stress of the shot-peening process can be imparted on thenitrogen diffusion layer. Therefore, the die-casting die havingexcellent abrasion resistance and excellent heat check resistance, basedon high hardness and high compression residual stress, can be produced.

Although the representative embodiments and their variations of thepresent invention are described herein, the present invention is notintended to be limited to them. Those skilled in the art could havefound various alternative embodiments and modifications withoutdeparture from the appended claims.

For example, although the foregoing explanation recites the specifiedshape and dimensions of each specimen, they are merely listed forconvenience of explanation and are not intended to limit the presentinvention.

BRIEF DESCRIPTIONS OF THE NUMERALS

-   1 Specimen-   2 Compound layer-   3 Nitrogen diffusion layer-   20 Examination equipment

1. A method for surface treatment of a die-casting die, which isprovided by applying the compression residual stress to an aestheticsurface of the die-casting die using a shot-peening process, the methodcomprising the steps of: nitride processing for forming on an aestheticsurface of the die-casting die a nitrided layer that includes at least acompound layer composed of a nitrogen compound by introducing gascontaining at least ammonia gas to a heating furnace; decomposing thecompound by exhausting the ammonia gas from the heating furnace andintroducing ambient gas to the heating furnace to carry out a thermalprocess to decompose the nitrogen compound; and shot-peening processingOR the aesthetic surface of the die; wherein the thickness of thecompound layer contained in the nitrogen layer that is formed in thestep of the nitriding process is within the range of from 2 μm to 7 μm.2. The method for surface treatment of a die-casting die of claim 1,wherein the step of decomposing the compound carries out the thermalprocess at a temperature that is lower than that of the step of thenitriding process.
 3. A die casting die to which the method for surfacetreatment of claim 1 or 2 is applied.